Various conventional forms of NOx sensors are known. Current state of the art compact mass-produced NOx sensors, as employed in the large IC engine and transportation industry, are extremely limited in variety, and are all based on solid-state electrochemical diffusion techniques that are, in turn, based on the original solid-state electrochemical oxygen sensor—a four-decade-old technology.
Since the following two patents issued—U.S. Pat. No. 4,770,760 (“Reduction, oxygen pumping and sensing cells, exhaust gases”) and U.S. Pat. No. 4,927,517 (“NOx sensor having catalyst for decomposing NOx”)—no significant market competition has emerged that supplants the current technology of mass-produced NOx sensors, consisting of diffusion based electrochemical detection methods. As a historical note, the aforementioned NOx sensors were, in turn, based on lambda (oxygen) sensors, specifically Robert Bosch G.M.B.H's 1976 patent U.S. Pat. No. 3,978,006 (“Methods for producing oxygen-sensing element, particularly for use with internal combustion engine exhaust emission analysis”).
Related art that has not yet achieved mass production or market acceptance, namely Nextech Materials, Ltd., with their patent application WO2013134738 A1, employs a method that bypasses the sequential diffusion steps and oxygen sensor detection techniques. Nextech accomplishes this by directly reducing NOx into oxygen ions that are subsequently injected into the amperometric device. While this is advancement over prior art, it is still a derivation of existing and extant techniques. In fact, it benefits from existing technology with oxygen migration through a solid electrolyte which in turn yields an amperometric response to oxygen ions which are electrically pumped through a specific material and subsequently measured, thereby inferring NOx concentration through an indirect means. But by doing this, Nextech Materials has overcome some (but not all) of the time delays inherent to systems based off the classic oxygen sensor.
In other mass produced related art, which represents virtually all of existing in situ NOx sensors for internal combustion applications, the concentration of NOx contained in a measurement gas (i.e. exhaust stream) is determined using primary and secondary electrochemical detecting elements, each of which includes an oxygen-pumping cell and an oxygen-sensing cell. Each of the pumping and sensing cells of the first and second detecting elements consists of a solid electrolyte and a pair of electrodes. At least one of the electrodes of the oxygen pumping and sensing cells of the second detecting elements which are exposed to the introduced measurement gas is used as a catalytic electrode which is manufactured with a catalyst, such as rhodium, for reducing the nitrogen oxides contained in the measurement gas. Oxygen is a reduction byproduct of NOx. Therefore the second detecting element (electrochemical oxygen sensor) can detect the oxygen partial pressure of the measurement gas generated as a result of the nitrogen oxides being reduced by the catalyst of the catalytic electrode. The NOx concentration is determined, based on a difference between two outputs of the first and second detecting elements which represent the oxygen partial pressures of respective portions of the measurement gas detected by the first and second detecting elements. The exact method of oxygen concentration measurement using this method can employ potentiometric mixed potential, impedance-based or, as mentioned with Nextech, amperometric sensor types.
The main problem with the conventional art stems primarily from the delays in measurement inherent to the design. Specifically, since mass produced NOx sensors are based off lambda (oxygen) sensors, each individual (and sequential) measurement within the multi-chambered sensor can take up to half a second. This is because the measurements take place in two separate chambers in the sensor, and the exhaust gases flow sequentially through one and then into the other. Gases flow from the exhaust stream through a diffusion barrier to reach the first chamber, which “pumps” the free oxygen out using a Nernst cell (simple oxygen sensor). The electric current for operating this first Nernst cell is used to measure exhaust oxygen content (lambda). With the free oxygen removed, the NOx is then left to migrate through another diffusion barrier and into the second measurement chamber. At this point, the NOx molecules encounter a catalytic element, which breaks them into nitrogen and oxygen gases. A second Nernst cell is then used to pump the newly generated oxygen out of the chamber, and this electric current is used to calculate the NOx levels in the exhaust. The residual nitrogen gases then flow out an exit port in the second measurement chamber. When combined with the diffusion barriers found within existing NOx sensor designs the overall measurement delay can be on the order of seconds. This process duration can be further increased as the system ages. While Nextech advanced the art, there is still a diffusion process of oxygen ions through a crystal substrate that accounts for their 50 millisecond response time.
Accordingly, there is a need in the art for improved devices and methods for in situ fluid composition and concentration detection/measurements including a mechanism or structural configuration to eliminate delays in such detection/measurements, and to provide an overall indication/determination of the efficiency of catalytic devices such as catalytic converters of internal combustion engines at a particular point in time.
Description of the Related Art Section Disclaimer: To the extent that specific patents/publications/products/systems are discussed above in this Background Section or elsewhere in this application, these discussions should not be taken as an admission that the discussed patents/publications/products/systems are prior art for patent law purposes. For example, some or all of the discussed patents/publications/products may not be sufficiently early in time, may not reflect subject matter developed early enough in time and/or may not be sufficiently enabling so as to amount to prior art for patent law purposes. To the extent that specific patents/publications/products/systems are discussed above in this Background Section and/or throughout the application, the descriptions/disclosures of which are all hereby incorporated by reference into this document in their respective entirety(ies).